1. Field of the Invention
The present invention relates to an image forming apparatus that performs electrophotographic image formation.
2. Description of the Related Art
Conventionally, there has been proposed an electrophotographic image forming apparatus that charges a rotating image carrier (photosensitive drum) using a charging section, exposes the same using an exposure section to form an electrostatic latent image on the surface of the photosensitive drum, develops the electrostatic latent image with toner, and transfers a toner image thus formed onto a recording material, to thereby form an image on the recording material.
For the electrophotographic image forming apparatus, there has been proposed a technique of correcting density unevenness in a toner image, caused by in-plane (surface) unevenness in potential characteristics (potential unevenness) on the photosensitive drum (see Japanese Patent Laid-Open Publication No. 2005-66827). According to the technique disclosed in Japanese Patent Laid-Open Publication No. 2005-66827, unevenness in electrical potential used for image formation, which will occur on the surface of the photosensitive drum during the image formation, is stored as data of electrical potential or data of density in the image forming apparatus in advance. Then, when performing exposure of the photosensitive drum by the exposure section, the exposure intensity is adjusted according to the data of electrical potential or the data of density, whereby the potential unevenness on the photosensitive drum is offset. Details of an example of control performed at the time will be given hereafter.
Referring first to FIG. 8, there is shown an example of a graph of potential unevenness on the photosensitive drum of the image forming apparatus. The potential unevenness is caused by in-plane unevenness which affects the easiness of charging when charging the photosensitive drum using the charging section, and unevenness in a drop in electrical potential which occurs with respect to a certain exposure intensity when the photosensitive drum is subjected to exposure using the exposure section. The graph shown in FIG. 8 will be explained hereinafter in the description of an embodiment of the present invention.
Referring next to FIG. 22, there is shown a distribution of the electrical potential on the photosensitive drum at each point on one line in a main scanning direction when the exposure section of the image forming apparatus scans the photosensitive drum along a direction of the axis of the photosensitive drum (in the main scanning direction) and thereby forms an electrostatic latent image in synchronism with rotation of the photosensitive drum. In a case where the electrical potential on the photosensitive drum after being subjected to charging and exposure suitable for image formation is 50V, as illustrated in FIG. 22, the exposure intensity is increased where the electrical potential is higher than 50V, and is lowered where the electrical potential is lower than 50V, according to potential characteristics detected when uniform charging and exposure is performed. This corrects uneven potentials into uniform potential.
In the image forming apparatus, the above-described correction is performed for each scan line when performing exposure of the photosensitive drum using the exposure section, whereby it becomes possible to correct potential unevenness on the whole photosensitive drum. Further, in correcting the potential unevenness on the photosensitive drum in a direction of rotation of the photosensitive drum i.e. in a sub scanning direction of the exposure section, based on the exposure intensity, it is necessary to control the rotational phase of the photosensitive drum, and at the same time, to change the exposure intensity according to the rotational phase.
One known method of controlling the rotational phase of the photosensitive drum uses a home position sensor. According to this method, the control is performed in the following manner: When performing image creation to form an electrostatic latent image on the photosensitive drum, the home position of the photosensitive drum is detected by the home position sensor when a certain time period required for stabilizing the rotation of the photosensitive drum elapses after the start of rotation of the photosensitive drum, and then, the rotational phase dependent on rotation performed starting from the time of detection of the home position is measured. According to the phase of the photosensitive drum thus controlled, the potential unevenness is corrected by changing the exposure intensity in the sub scanning direction, similarly to the potential unevenness correction in the main scanning direction.
Further, there has been proposed a method of, when a photosensitive drum is made in a manufacturing plant, measuring the above-mentioned potential unevenness on the photosensitive drum in advance and storing data of the measurement, which is formed as data defined with reference to a phase reference position on the photosensitive drum as a starting point, in a storage section of an image forming apparatus on which the photosensitive drum is mounted.
On the other hand, there has been known an image forming apparatus that produces a printout by adding tiny dots to an original image, and when copying the printout, determines whether the printout is permitted to be copied according to a usage restriction expressed by a pattern of the tiny dots (see Japanese Patent Laid-Open Publication No. H08-130626).
To stably read information expressed by the tiny dots during copying, it is important to cause the tiny dots added to the original image to be uniformly reproduced on an image surface. Therefore, it is necessary to correct the potential unevenness on the photosensitive drum during image formation which causes unevenness of the reproducibility of the tiny dots on the image surface.
However, if the home position of the photosensitive drum of the image forming apparatus does not coincide with the phase reference position on the photosensitive drum, the profile of the exposure intensity switching does not coincide with the actual potential unevenness on the photosensitive drum, and hence there is a high possibility that the potential unevenness is further increased. Particularly, in a case where the photosensitive drum and a member (flange) used for detecting the home position of the photosensitive drum are different members, unless the accuracy is high with which the two members are mounted to each other to form a unit, there is a high possibility that the potential unevenness is increased.
To solve such a problem, if respective correct mounting positions of the photosensitive drum and the member mounted to the photosensitive drum are searched for while performing image formation by the image forming apparatus, it is required to remove the photosensitive drum from the image forming apparatus each time to adjust the mounting positions. Particularly, the efficiency is largely lowered in a maintenance operation in which the photosensitive drum is often singly subjected to component replacement after removal from the unit formed by the photosensitive drum and accessories mounted thereon.